1. Field of the Invention
This invention relates in general to magnetic tunnel junction transducers for reading information signals from a magnetic medium and, in particular, to a magnetic tunnel junction magnetoresistive sensor with an improved bias layer structure.
2. Description of the Related Art
Computers often include auxiliary memory storage devices having media on which data can be written and from which data can be read for later use. A direct access storage device (disk drive) incorporating rotating magnetic disks is commonly used for storing data in magnetic form on the disk surfaces. Data is recorded on concentric, radially spaced tracks on the disk surfaces. Magnetic heads including read sensors are then used to read data from the tracks on the disk surfaces.
In high capacity disk drives, magnetoresistive (MR) read sensors, commonly referred to as MR sensors, are the prevailing read sensors because of their capability to read data from a surface of a disk at greater track and linear densities than thin film inductive heads. An MR sensor detects a magnetic field through the change in the resistance of its MR sensing layer (also referred to as an xe2x80x9cMR elementxe2x80x9d) as a function of the strength and direction of the magnetic flux being sensed by the MR layer.
The conventional MR sensor operates on the basis of the anisotropic magnetoresistive (AMR) effect in which an MR element resistance varies as the square of the cosine of the angle between the magnetization in the MR element and the direction of sense current flowing through the MR element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the MR element, which in turn causes a change in resistance in the MR element and a corresponding change in the sensed current or voltage.
Another type of MR sensor is the giant magnetoresistance (GMR) sensor manifesting the GMR effect. In GMR sensors, the resistance of the MR sensing layer varies as a function of the spin-dependent transmission of the conduction electrons between magnetic layers separated by a non-magnetic layer (spacer) and the accompanying spin-dependent scattering which takes place at the interface of the magnetic and non-magnetic layers and within the magnetic layers.
GMR sensors using only two layers of ferromagnetic material (e.g., Nixe2x80x94Fe) separated by a layer of non-magnetic material (e.g., copper) are generally referred to as spin valve (SV) sensors manifesting the SV effect. FIG. 1 shows a prior art SV sensor 100 comprising end regions 104 and 106 separated by a central region 102. A first ferromagnetic layer, referred to as a pinned layer 120, has its magnetization typically fixed (pinned) by exchange coupling with an antiferromagnetic (AFM) layer 125. The magnetization of a second ferromagnetic layer, referred to as a free layer 110, is not fixed and is free to rotate in response to the magnetic field from the recorded magnetic medium (the signal field). The free layer 110 is separated from the pinned layer 120 by a non-magnetic, electrically conducting spacer layer 115. Leads 140 and 145 formed in the end regions 104 and 106, respectively, provide electrical connections for sensing the resistance of SV sensor 100. IBM""s U.S. Pat. No. 5,206,590 granted to Dieny et al., incorporated herein by reference, discloses a SV sensor operating on the basis of the GMR effect.
Another type of SV sensor is an antiparallel (AP)-pinned SV sensor. In AP-pinned SV sensors, the pinned layer is a laminated structure of two ferromagnetic layers separated by a non-magnetic coupling layer such that the magnetizations of the two ferromagnetic layers are strongly coupled together antiferromagnetically in an antiparallel orientation. The AP-pinned SV sensor provides improved exchange coupling of the antiferromagnetic (AFM) layer to the laminated pinned layer structure than is achieved with the pinned layer structure of the SV sensor of FIG. 1. This improved exchange coupling increases the stability of the AP-pinned SV sensor at high temperatures which allows the use of corrosion resistant and electrically insulating antiferromagnetic materials such as NiO for the AFM layer.
Another type-of magnetic device currently under development is a magnetic tunnel junction (MTJ) device. The MTJ device has potential applications as a memory cell and as a magnetic field sensor. The MTJ device comprises two ferromagnetic layers separated by a thin, electrically insulating, tunnel barrier layer. The tunnel barrier layer is sufficiently thin that quantum-mechanical tunneling of charge carriers occurs between the ferromagnetic layers. The tunneling process is electron spin dependent, which means that the tunneling current across the junction depends on the spin-dependent electronic properties of the ferromagnetic materials and is a function of the relative orientation of the magnetic moments, or magnetization directions, of the two ferromagnetic layers. In the MTJ sensor, one ferromagnetic layer has its magnetization fixed, or pinned, and the other ferromagnetic layer has its magnetization free to rotate in response to an external magnetic field from the recording medium (the signal field). When an electric potential is applied between the two ferromagnetic layers, the sensor resistance is a function of the tunneling current across the insulating layer between the ferromagnetic layers. Since the tunneling current that flows perpendicularly through the tunnel barrier layer depends on the relative magnetization directions of the two ferromagnetic layers, recorded data can be read from a magnetic medium because the signal field causes a change of direction of magnetization of the free layer, which in turn causes a change in resistance of the MTJ sensor and a corresponding change in the sensed current or voltage. IBM""s U.S. Pat. No. 5,650,958 granted to Gallagher et al., incorporated in its entirety herein by reference, discloses an MTJ sensor operating on the basis of the magnetic tunnel junction effect.
FIG. 2 shows a prior art MTJ sensor 200 comprising a first electrode 204, a second electrode 202, and a tunnel barrier 215. The first electrode 204 comprises a pinned layer (pinned ferromagnetic layer) 220, an antiferromagnetic (AFM) layer 230, and a seed layer 240. The magnetization of the pinned layer 220 is fixed through exchange coupling with the AFM layer 230. The second electrode 202 comprises a free layer (free ferromagnetic layer) 210 and a cap layer 205. The free layer 210 is separated from the pinned layer 220 by a non-magnetic, electrically insulating tunnel barrier layer 215. In the absence of an external magnetic field, the free layer 210 has its magnetization oriented in the direction shown by arrow 212, that is, generally perpendicular to the magnetization direction of the pinned layer 220 shown by arrow 222 (tail of the arrow that is pointing into the plane of the paper). A first lead 260 and a second lead 265 formed in contact with first electrode 204 and second electrode 202, respectively, provide electrical connections for the flow of sensing current Is from a current source 270 to the MTJ sensor 200. A signal detector 280, typically including a recording channel such as a partial-response maximum-likelihood (PRML) channel, connected to the first and second leads 260 and 265 senses the change in resistance due to changes induced in the free layer 210 by the external magnetic field.
As mentioned earlier, an MR sensor exhibits a change in resistance when in the presence of a changing magnetic field. This resistance change is transformed into a voltage signal by passing a constant sense current through the MR element. The value of the DC voltage for a given MR sensor is the product of the constant sense current and the total resistance between the MR sensor leads. Since the change in the resistance is the principal upon which the MR sensor operates, the change in resistance can substantially effect the performance of the MR sensor and the disk drive incorporating the MR sensor.
The transfer curve (readback signal of the MTJ head versus applied signal from the magnetic disk) for an MTJ sensor is linear and is defined by sin xcex8 where xcex8 is the angle between the directions of the magnetizations of the free and pinned layers. FIG. 3a is an exemplary transfer curve for an MTJ sensor having a bias point (operating point) 300 at the midpoint of the transfer curve, at which point the positive and negative readback signals V1 and V2 (positive and negative changes in the GMR of the MTJ sensor above and below the bias point) are equal (symmetrical) when sensing positive and negative fields having the same magnitude from the magnetic disk. FIGS. 3b and 3c illustrate transfer curves having bias points 302 and 304 shifted in the positive and negative directions, respectively, so that the readback signals V1 and V2 are asymmetrical with respect to the bias point.
The desirable symmetric bias transfer curve of FIG. 3a is obtained when the MTJ sensor is in its quiescent state (no magnetic signal from the disk) and the direction of the magnetization of the free layer is perpendicular to the magnetization of the pinned layer which is fixed substantially perpendicular to the disk surface. The bias point may be shifted from the midpoint of the transfer curve by various influences on the free layer which in the quiescent state can act to rotate its magnetization relative to the magnetization of the pinned layer.
The bias point is influenced by two major forces on the free layer, namely a ferromagnetic coupling field HFC between the pinned layer and the free layer, and a demagnetization field Hdemag on the free layer from the pinned layer. In a conventional spin valve sensor these coupling fields may be partially offset by the self-field provided by the sense current passing parallel to the layers of the sensor. In an MTJ sensor the current passes perpendicular to the sensor and cannot be used to provide such a field. IBM""s U.S. Pat. No. 6,023,395 to Dill et al., incorporated herein by reference, discloses an MTJ sensor with in-stack biasing to stabilize the sensor and to provide bias point symmetry to linearize the output of the sensor.
As data storage density in disk drives increases, the need for thinner read sensors makes the use of AP-pinned MTJ sensors desirable since the AP-pinned structure promotes a stronger exchange interaction with the AFM layer allowing thinner AFM layers to be used. Therefore there is a need for an AP-pinned MTJ sensor with an improved transverse biasing structure to provide bias point symmetry with improved manufacturability.
Accordingly, it is an object of the present invention to disclose a antiparallel (AP)-pinned magnetic tunnel junction (MTJ) sensor having a free (sensing) layer, an AP-pinned layer, an electrically non-conductive tunnel barrier layer sandwiched between the free layer and the AP-pinned layer, a bias layer, and an antiferromagnetic (AFM) layer disposed between the AP-pinned layer and the bias layer.
It is another object of the present invention to disclose an AP-pinned MTJ sensor having a single AFM layer to pin the magnetizations of a pinned layer and of a bias layer for providing a transverse bias field at a free layer.
It is a further object of the present invention to disclose an AP-pinned MTJ sensor having an AP-pinned bias layer wherein a single AFM layer pins the magnetizations of the pinned layer and the bias layer.
It is yet a further object of the present invention to disclose an AP-pinned MTJ sensor having a bias layer to provide a transverse magnetic field at the free layer for obtaining zero or nearly zero signal asymmetry.
In accordance with the principles of the present invention, there is disclosed a preferred embodiment of the present invention wherein an AP-pinned MTJ sensor has a thin AFM layer and an AP-pinned layer with first and second ferromagnetic layers of different thickness coupled by an antiparallel coupling (APC) layer. The first ferromagnetic (FM1) layer is adjacent to the AFM layer and the thicker second ferromagnetic (FM2) layer is adjacent to an electrically insulating (non-conductive) tunnel barrier layer. A ferromagnetic free layer is adjacent to the spacer layer. An AP-pinned bias layer with first and second ferromagnetic bias layers of different thickness coupled by an APC layer is located on the side of the AFM layer opposite to the side adjacent to the AP-pinned layer. The second bias (B2) layer adjacent to the AFM layer is not as thick as the first bias (B1) layer.
Because the FM2 layer of the AP-pinned layer and the B1 layer of the AP-pinned bias layer are thicker than the FM1 and the B2 layers, respectively, the net magnetization of the AP-pinned layer is in the same direction as the net magnetization of the AP-pinned bias layer. A net demagnetization field HdmP from the AP-pinned layer acts on the free layer in the same direction as a net demagnetization field HdmB from the AP-pinned bias layer. The combined demagnetization fields HdmP+HdmB are directed opposite to and oppose a ferromagnetic coupling field HFC from the FM2 layer of the AP-pinned layer acting on the free layer. The relative thicknesses of the FM1 and FM2 layers and the B1 an B2 layers, respectively, are chosen so that the sum of the demagnetization fields HdmP+HdmB counterbalance the ferromagnetic coupling field HFC to obtain zero or near zero asymmetry of the bias point on the transfer curve of the MTJ sensor.
Having a single AFM layer pinning the magnetization directions of both the AP-pinned layer and the bias layer provides a further advantage in improving the manufacturability of the MTJ sensor by only requiring setting the magnetization of a single AFM material during the fabrication process.
The above as well as additional objects, features, and advantages of the present invention will become apparent in the following detailed description.